Remove file_mode::append_new (API Change):

* Tidying
* Increase test coverage
* Fix file_mode::append_existing

API Changes:

* file_mode::append_new is removed, as it makes no sense

Actions Required:

* Replace file_mode::append_new with file_mode::append
  or file_mode::append_existing instead of file_mode::append_new

test/beast/core/file_stdio.cpp

@@ -27,7 +27,7 @@ public:
     void
     run()
     {
-        doTestFile<file_stdio>(*this);
+        test_file<file_stdio>(*this);
     }
 };